An important procedural aspect of the manufacture of filter assemblies is the process of bonding an end cap to a filter pack. As used herein, the term "filter pack" refers to any structure that includes a filter medium. The filter medium may be, but is not limited to, a metal mesh or a porous metal medium formed from metal powder or metal fibers.
One conventional method of bonding is known as brazing. The end of a filter pack is placed in an end cap with a brazing alloy and the resultant assembly is heated until the brazing alloy melts. The assembly is then cooled and as the molten brazing alloy solidifies, it bonds the end cap to the filter pack.
One disadvantage of this brazing method is that when the brazing alloy melts, the molten brazing alloy may wick up into or along the filter pack for a significant distance away from the end cap. This sometimes starves the joint between the filter pack and the end cap for brazing alloy, resulting in incomplete bonding. In addition, when the molten brazing alloy solidifies, it then blinds the porous filter medium, i.e., closes off the pores of the filter medium to fluid flow. This significantly decreases the effective filtration area of the filter medium and reduces the efficiency of the filter assembly.
Another disadvantage of this brazing method becomes apparent when the brazing method is applied to fine fiber metal media. As the brazing alloy melts, alloying between the fibers and the brazing alloy causes the fibers to melt thus creating a defective seal.
In another conventional method of bonding, the end of a filter pack is compressed to form a dense area. A stainless steel ring and the compressed end of the filter pack are then positioned in an end cap with a brazing alloy. The entire assembly is then welded together to form a filter assembly.
This method gives rise to several disadvantages. Special tooling is required which renders this method undesirably expensive. By compressing the ends of the filter pack, the effective filtration area is reduced, thereby decreasing the efficiency of the filter assembly. Compressing the ends of the filter pack may also cause damage to the filter medium, creating leaks which are difficult to repair. This is a problem especially associated with pleated filter assemblies having wide, open pleats. Further, welding the filter pack to the end cap frequently results in discoloration of the filter assembly.
Another important disadvantage associated with compressing the filter pack prior to welding becomes apparent in the cleaning process. During normal use, the filter assemblies tend to become clogged and require cleaning. Commonly used cleaning agents such as nitric acid may attack the brazing alloy. Thus, the cleaning agent used to clean the filter assemblies must be carefully selected so as not to cause damage to the brazing alloy. In addition, the densified areas of the filter assembly that result from compression have proven to be especially difficult to clean as carbon deposits tend to accumulate in such areas. If these carbon deposits are released during subsequent use, they may contaminate the filtrate.